About unusually large low-level CAPE and supercell tornadoes
When reliable observations or estimates of 0-3 km CAPE approach around 200 J/kg or larger, F1 or F2 intensity tornadoes can occur with relatively weak SRH,vertical shear, and shear-CAPE combinations, particularly if there are pre-existing boundaries nearby. In the RUC-2 database profiles that were examined, only 15 of 321cases (5%) had 0-3 km CAPE of 200 J/kg or greater, but 13 of these cases (87%) produced tornadoes. When low-level CAPE is this large, CIN will typically be quite small.
Below is the Eta model analysis environment (updated by local surface
observations) for the Dallas-Ft. Worth area at early afternoon on 9/5/01. This was
associated with a "surprise" F1 tornado that did minor damage in the northern
Dallas metropolitan area at Carollton, Texas.
Notice the large 0-3 km CAPE (> 200 J/kg) and very small CIN, even though SRH and
vertical wind shear are weak. The weak tornado occurred with a small storm having
some supercell characteristics that was associated with a northward moving midlevel
vorticity maximum and surface boundary (not shown).
Here is another example showing the Eta model analysis environment at
Houston early morning on 10/11/01,associated with a tornado (3 mile path) that produced F1
and marginal F2 damage near Pasadena, Texas:
This profile also has large 0-3 km CAPE (> 200 J/kg) and almost no CIN.
Vertical shear and SRH are larger than on the previous example, but still relatively weak.
This last example is a RUC-2 profile at Washington, D.C., mid afternoon
on 9/24/01associated with an F3 tornado that killed 2 people in Maryland just northeast of
the District of Columbia:
Similar to the previous 2 examples, this profile has nearly 200 J/kg of 0-3
km CAPE and small CIN. But in this case, SRH and vertical shear are notably large
with values more typically associated with strong tornadoes. Also notice how
saturated this profile is through midlevels, showing that dry air aloft isn't necessary
for strong tornadoes.
Small supercells or mini-supercells that produce tornadoes will typically have around 200 J/kg or more of CAPE below 3 km. See tornadic mini-supercell example for a situation with extreme 0-3 km CAPE (around 300 J/kg).
A cautionary note using model profiles to compute 0-3 km CAPE: Model forecasts of this parameter tend to be exaggerated and inflated due to errors in boundary-layer forecasts and the sensitivity of low-level CAPE to these errors, particularly in the eastern 1/3rd of the U.S. and along coastal areas. Always update model data with actual surface observations when possible, and cross-reference low-level CAPE with CIN and LFC computations.
back to operational low-level buoyancy parameter guidelines